Magnetic bubble memories are well known in the art. Bubble propagation is accomplished in such memories by creating a changing pattern of localized magnetic field gradients. U.S. Pat. No. 3,534,347 of A. H. Bobeck issued Oct. 13, 1970 discloses a "field-access" mode bubble memory in which the localized field gradients are produced in a pattern of magnetically soft elements adjacent the bubble layer by a magnetic field rotating in the plane of bubble movement. The elements, typically of Permalloy, have a geometry and orientation such that the rotating in-plane field produces in them consecutively offset positions magnetic poles which attract bubbles and which are operative to move bubbles along a path defined by the elements.
A variety of geometries has been used for the propagation elements of Permalloy field-access bubble memories. The T-bar geometry, disclosed in the above-mentioned patent, was one of the original circuit patterns used. That pattern employs elements oriented at 90 degrees to one another so that each 90 degree rotation of the in-plane field moves the bubble to a new position. Efforts to improve the operating margins of bubble memories have resulted in a number of alternative geometries, including the Y-bar, the asymmetric half-disc, and the presently used asymmetric chevron.
Whatever the geometry of the elements, the propagation path is formed by a series of the elements oriented in such a manner that due to the rotation of the in-plane field and consequent pole formation, a bubble moves to successive positions on an element, and at a certain point, again due to the geometry of the pole formation, transfers to the next adjacent element. In the asymmetric disc or chevron pattern, the bubble lingers near the end of the instant element until it sees a stronger attractive pole on the next adjacent element and then transfers to that element. It is thought to be advantageous for the pole formed on the element to which the bubble transfers to be strong enough to "pull" the bubble over to complete the transfer. This requirement limits the element design of the propagation circuit.
For operative bubble memories, the gap between adjacent elements is typically smaller than the bubble "collapse diameter" and is the limiting feature in bubble memory design. The problem to which this invention is directed thus is to relax the requirements on the gap between adjacent elements in magnetic bubble memories.